A switching power supply employing a switching element is widely used in various fields as a power supply suitable for power saving and downsizing. As an example, the switching power supply is used also as a power supply for illumination. In recent years, replacement of lighting sources from incandescent lamps or fluorescent lamp to energy saving and long life light sources such as a light-emitting diode (LED) is in progress. Also, for example, new lighting sources such as Organic light-emitting diode (OLED) are also developed.
However, heat generation density of the switching element or the like is increased if the switching power supply is increased in frequency and downsized.